Tanerreported this bug
MythicMobDespawnEvent is sometimes triggered together with MythicMobDeathEvent, even though it should not be.
- Create a spawner in the middle of a chunk and set its timers to low values. Example spawner and mob configurations: https://mclo.gs/NOm8fSW
- Restart the server while you are in the same chunk as the spawner and mob is spawned already. After rejoining the server, kill the mob. Sometimes you may need to kill it multiple times or restart the server while the mob is still spawned.
I found that this makes the bug much easier to reproduce. Under normal circumstances, it happens randomly on servers with hundreds of spawners.
MythicMobDespawnEvent should only be fired when a mob actually despawns.
In the video below, you can see that everything works correctly when the server is started for the first time. However, after the server is restarted and opened again, the DespawnEvent is triggered once more.